Android 提供了AnimationDrawable用于實(shí)現(xiàn)幀動(dòng)畫(huà)蛾派。在動(dòng)畫(huà)開(kāi)始之前俄认，所有幀的圖片都被解析并占用內(nèi)存个少，一旦動(dòng)畫(huà)較復(fù)雜幀數(shù)較多，在低配置手機(jī)上容易發(fā)生 OOM眯杏。即使不發(fā)生 OOM夜焦，也會(huì)對(duì)內(nèi)存造成不小的壓力。下面代碼展示了一個(gè)幀數(shù)為4的幀動(dòng)畫(huà)：

原生幀動(dòng)畫(huà)

AnimationDrawable drawable = new AnimationDrawable();
drawable.addFrame(getDrawable(R.drawable.frame1), frameDuration);
drawable.addFrame(getDrawable(R.drawable.frame2), frameDuration);
drawable.addFrame(getDrawable(R.drawable.frame3), frameDuration);
drawable.addFrame(getDrawable(R.drawable.frame4), frameDuration);
drawable.setOneShot(true);

ImageView ivFrameAnim = ((ImageView) findViewById(R.id.frame_anim));
ivFrameAnim.setImageDrawable(drawable);
drawable.start();

有沒(méi)有什么辦法讓幀動(dòng)畫(huà)的數(shù)據(jù)逐幀加載役拴，而不是一次性全部加載到內(nèi)存糊探？SurfaceView就提供了這種能力钾埂。

SurfaceView

屏幕的顯示機(jī)制和幀動(dòng)畫(huà)類(lèi)似河闰，也是一幀一幀的連環(huán)畫(huà)，只不過(guò)刷新頻率很高褥紫，感覺(jué)像連續(xù)的姜性。為了顯示一幀，需要經(jīng)歷計(jì)算和渲染兩個(gè)過(guò)程髓考，CPU 先計(jì)算出這一幀的圖像數(shù)據(jù)并寫(xiě)入內(nèi)存部念，然后調(diào)用 OpenGL 命令將內(nèi)存中數(shù)據(jù)渲染成圖像存放在 GPU Buffer 中，顯示設(shè)備每隔一定時(shí)間從 Buffer 中獲取圖像并顯示氨菇。

上述過(guò)程中的計(jì)算儡炼，對(duì)于View來(lái)說(shuō)，就好比在主線程遍歷 View樹(shù) 以決定視圖畫(huà)多大（measure）查蓉，畫(huà)在哪（layout）乌询，畫(huà)些啥（draw），計(jì)算結(jié)果存放在內(nèi)存中豌研，SurfaceFlinger 會(huì)調(diào)用 OpenGL 命令將內(nèi)存中的數(shù)據(jù)渲染成圖像存放在 GPU Buffer 中妹田。每隔16.6ms，顯示器從 Buffer 中取出幀并顯示鹃共。所以自定義 View 可以通過(guò)重載onMeasure()鬼佣、onLayout()、onDraw()來(lái)定義幀內(nèi)容霜浴，但不能定義幀刷新頻率晶衷。

SurfaceView可以突破這個(gè)限制。而且它可以將計(jì)算幀數(shù)據(jù)放到獨(dú)立的線程中進(jìn)行阴孟。下面是自定義SurfaceView的模版代碼：

public abstract class BaseSurfaceView extends SurfaceView implements SurfaceHolder.Callback {
    public static final int DEFAULT_FRAME_DURATION_MILLISECOND = 50;
    //用于計(jì)算幀數(shù)據(jù)的線程
    private HandlerThread handlerThread;
    private Handler handler;
    //幀刷新頻率
    private int frameDuration = DEFAULT_FRAME_DURATION_MILLISECOND;
    //用于繪制幀的畫(huà)布
    private Canvas canvas;
    private boolean isAlive;

    public BaseSurfaceView(Context context) {
        super(context);
        init();
    }

    protected void init() {
        getHolder().addCallback(this);
        //設(shè)置透明背景房铭，否則SurfaceView背景是黑的
        setBackgroundTransparent();
    }

    private void setBackgroundTransparent() {
        getHolder().setFormat(PixelFormat.TRANSLUCENT);
        setZOrderOnTop(true);
    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        isAlive = true;
        startDrawThread();
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        stopDrawThread();
        isAlive = false;
    }

    //停止幀繪制線程
    private void stopDrawThread() {
        handlerThread.quit();
        handler = null;
    }

    //啟動(dòng)幀繪制線程
    private void startDrawThread() {
        handlerThread = new HandlerThread("SurfaceViewThread");
        handlerThread.start();
        handler = new Handler(handlerThread.getLooper());
        handler.post(new DrawRunnable());
    }

    private class DrawRunnable implements Runnable {

        @Override
        public void run() {
            if (!isAlive) {
                return;
            }
            try {
                //1.獲取畫(huà)布
                canvas = getHolder().lockCanvas();
                //2.繪制一幀
                onFrameDraw(canvas);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                //3.將幀數(shù)據(jù)提交
                getHolder().unlockCanvasAndPost(canvas);
                //4.一幀繪制結(jié)束
                onFrameDrawFinish();
            }
            //不停的將自己推送到繪制線程的消息隊(duì)列以實(shí)現(xiàn)幀刷新
            handler.postDelayed(this, frameDuration);
        }
    }
    
    protected abstract void onFrameDrawFinish();

    protected abstract void onFrameDraw(Canvas canvas);
}

• 用HandlerThread作為獨(dú)立幀繪制線程，好處是可以通過(guò)與其綁定的Handler方便地實(shí)現(xiàn)“每隔一段時(shí)間刷新”温眉，而且在Surface被銷(xiāo)毀的時(shí)候可以方便的調(diào)用HandlerThread.quit()來(lái)結(jié)束線程執(zhí)行的邏輯缸匪。
• DrawRunnable.run()運(yùn)用模版方法模式定義了繪制算法框架，其中幀繪制邏輯的具體實(shí)現(xiàn)被定義成兩個(gè)抽象方法类溢，推遲到子類(lèi)中實(shí)現(xiàn)凌蔬。本文的主角FrameSurfaceView應(yīng)該繼承自BaseSurfaceView：

逐幀解析 & 及時(shí)回收

public class FrameSurfaceView extends BaseSurfaceView {
    public static final int INVALID_BITMAP_INDEX = Integer.MAX_VALUE;
    private List<Integer> bitmaps = new ArrayList<>();
    //幀圖片
    private Bitmap frameBitmap;
    //幀索引
    private int bitmapIndex = INVALID_BITMAP_INDEX;
    private Paint paint = new Paint();
    private BitmapFactory.Options options = new BitmapFactory.Options();
    //幀圖片原始大小
    private Rect srcRect;
    //幀圖片目標(biāo)大小
    private Rect dstRect = new Rect();
    private int defaultWidth;
    private int defaultHeight;

    public void setDuration(int duration) {
        int frameDuration = duration / bitmaps.size();
        setFrameDuration(frameDuration);
    }

    public void setBitmaps(List<Integer> bitmaps) {
        if (bitmaps == null || bitmaps.size() == 0) {
            return;
        }
        this.bitmaps = bitmaps;
        //默認(rèn)情況下露懒，計(jì)算一幀圖片的原始大小
        getBitmapDimension(bitmaps.get(0));
    }
    
    private void getBitmapDimension(Integer integer) {
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(this.getResources(), integer, options);
        defaultWidth = options.outWidth;
        defaultHeight = options.outHeight;
        srcRect = new Rect(0, 0, defaultWidth, defaultHeight);
        requestLayout();
    }

    public FrameSurfaceView(Context context) {
        super(context);
    }
    
    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        dstRect.set(0, 0, getWidth(), getHeight());
    }

    @Override
    protected void onFrameDrawFinish() {
        //在一幀繪制完后，直接回收它
        recycleOneFrame();
    }

    //回收幀
    private void recycleOneFrame() {
        if (frameBitmap != null) {
            frameBitmap.recycle();
            frameBitmap = null;
        }
    }

    @Override
    protected void onFrameDraw(Canvas canvas) {
        //繪制一幀前需要先清畫(huà)布砂心，否則所有幀都疊在一起同時(shí)顯示
        clearCanvas(canvas);
        if (!isStart()) {
            return;
        }
        if (!isFinish()) {
            drawOneFrame(canvas);
        } else {
            onFrameAnimationEnd();
        }
    }

    //繪制一幀懈词，是張Bitmap
    private void drawOneFrame(Canvas canvas) {
        frameBitmap = BitmapUtil.decodeOriginBitmap(getResources(), bitmaps.get(bitmapIndex), options);
        canvas.drawBitmap(frameBitmap, srcRect, dstRect, paint);
        bitmapIndex++;
    }

    private void onFrameAnimationEnd() {
        reset();
    }

    private void reset() {
        bitmapIndex = INVALID_BITMAP_INDEX;
    }

    //幀動(dòng)畫(huà)是否結(jié)束
    private boolean isFinish() {
        return bitmapIndex >= bitmaps.size();
    }

    //幀動(dòng)畫(huà)是否開(kāi)始
    private boolean isStart() {
        return bitmapIndex != INVALID_BITMAP_INDEX;
    }
    
    //開(kāi)始播放幀動(dòng)畫(huà)
    public void start() {
        bitmapIndex = 0;
    }

    private void clearCanvas(Canvas canvas) {
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        canvas.drawPaint(paint);
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
    }
}

• FrameSurfaceView繼承自BaseSurfaceView，所以它復(fù)用了基類(lèi)的繪制框架算法辩诞，并且定了自己每一幀的繪制內(nèi)容：一張Bitmap坎弯。
• Bitmap資源 id 通過(guò)setBitmap()傳遞進(jìn)來(lái)， 繪制一幀解析一張 译暂，在每一幀繪制完畢后抠忘，調(diào)用Bitmap.recycle()釋放圖片 native 內(nèi)存并去除 java 堆中圖片像素?cái)?shù)據(jù)的引用。這樣當(dāng) GC 發(fā)生時(shí)外永，圖片像素?cái)?shù)據(jù)可以及時(shí)被回收崎脉。

一切都是這么地能夠自圓其說(shuō)，我迫不及待地運(yùn)行代碼并打開(kāi)AndroidStudio的Profiler標(biāo)簽頁(yè)伯顶，切換到MEMORY囚灼，想用真實(shí)內(nèi)存數(shù)據(jù)驗(yàn)證下性能。但殘酷的事實(shí)狠狠地打了下臉祭衩。灶体。。多次播放幀動(dòng)畫(huà)后掐暮，內(nèi)存占用居然比原生AnimationDrawable還大蝎抽，而且每播放一次，內(nèi)存中都會(huì)多出 N 個(gè)Bitmap對(duì)象（N為播放一邊的幀數(shù)）劫乱。唯一令人欣慰的是织中，手動(dòng)觸發(fā) GC 后幀動(dòng)畫(huà)圖片能夠被回收。（AnimationDrawable中的圖片數(shù)據(jù)不會(huì)被 GC）

原因就在于自作聰明地及時(shí)回收衷戈，每一幀繪制完后幀數(shù)據(jù)被回收狭吼，那下一幀解析Bitmap時(shí)只能新申請(qǐng)一塊內(nèi)存。幀動(dòng)畫(huà)每張圖片大小是一致的殖妇，是不是能復(fù)用上一幀Bitmap的內(nèi)存空間刁笙？于是乎有了下面這個(gè)版本的FrameSurfaceView：

逐幀解析 & 幀復(fù)用

public class FrameSurfaceView extends BaseSurfaceView {
    public static final int INVALID_BITMAP_INDEX = Integer.MAX_VALUE;
    private List<Integer> bitmaps = new ArrayList<>();
    private Bitmap frameBitmap;
    private int bitmapIndex = INVALID_BITMAP_INDEX;
    private Paint paint = new Paint();
    private BitmapFactory.Options options;
    private Rect srcRect;
    private Rect dstRect = new Rect();

    public void setDuration(int duration) {
        int frameDuration = duration / bitmaps.size();
        setFrameDuration(frameDuration);
    }

    public void setBitmaps(List<Integer> bitmaps) {
        if (bitmaps == null || bitmaps.size() == 0) {
            return;
        }
        this.bitmaps = bitmaps;
        getBitmapDimension(bitmaps.get(0));
    }

    private void getBitmapDimension(Integer integer) {
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(this.getResources(), integer, options);
        defaultWidth = options.outWidth;
        defaultHeight = options.outHeight;
        srcRect = new Rect(0, 0, defaultWidth, defaultHeight);;
    }

    public FrameSurfaceView(Context context) {
        super(context);
    }

    @Override
    protected void init() {
        super.init();
        //定義解析Bitmap參數(shù)為可變類(lèi)型，這樣才能復(fù)用Bitmap
        options = new BitmapFactory.Options();
        options.inMutable = true;
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        dstRect.set(0, 0, getWidth(), getHeight());
    }

    @Override
    protected int getDefaultWidth() {
        return defaultWidth;
    }

    @Override
    protected int getDefaultHeight() {
        return defaultHeight;
    }

    @Override
    protected void onFrameDrawFinish() {
        //每幀繪制完畢后不再回收
//        recycle();
    }

    public void recycle() {
        if (frameBitmap != null) {
            frameBitmap.recycle();
            frameBitmap = null;
        }
    }

    @Override
    protected void onFrameDraw(Canvas canvas) {
        clearCanvas(canvas);
        if (!isStart()) {
            return;
        }
        if (!isFinish()) {
            drawOneFrame(canvas);
        } else {
            onFrameAnimationEnd();
        }
    }

    private void drawOneFrame(Canvas canvas) {
        frameBitmap = BitmapUtil.decodeOriginBitmap(getResources(),
        //復(fù)用Bitmap
        bitmaps.get(bitmapIndex), options);
        options.inBitmap = frameBitmap;
        canvas.drawBitmap(frameBitmap, srcRect, dstRect, paint);
        bitmapIndex++;
    }

    private void onFrameAnimationEnd() {
        reset();
    }

    private void reset() {
        bitmapIndex = INVALID_BITMAP_INDEX;
    }

    private boolean isFinish() {
        return bitmapIndex >= bitmaps.size();
    }

    private boolean isStart() {
        return bitmapIndex != INVALID_BITMAP_INDEX;
    }

    public void start() {
        bitmapIndex = 0;
    }

    private void clearCanvas(Canvas canvas) {
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        canvas.drawPaint(paint);
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
    }
}

• 將Bitmap的解析參數(shù)inBitmap設(shè)置為已經(jīng)成功解析的Bitmap對(duì)象以實(shí)現(xiàn)復(fù)用谦趣。

這一次不管重新播放多少次幀動(dòng)畫(huà)疲吸，內(nèi)存中Bitmap數(shù)量只會(huì)增加1，因?yàn)橹辉诮馕龅谝粡垐D片是分配了內(nèi)存前鹅。而這塊內(nèi)存可以在FrameSurfaceView生命周期結(jié)束時(shí)手動(dòng)調(diào)用recycle()回收摘悴。

talk is cheap, show me the code

為了更清晰的展示祟同，上述代碼段省略了一些和主題無(wú)關(guān)的自定義 View 細(xì)節(jié)瞧剖，完整的代碼可以點(diǎn)擊這里痹筛。